Valves are known for use in systems for low pressure metering and delivery of components of a liquid composition in systems, such as HPLC systems, that require accurate proportioning of components of the liquid composition. U.S. Pat. No. 4,595,496 ("the '496 patent"), which is hereby incorporated herein by reference, describes various known implementations for addressing applications which require mixing liquids in controlled proportions. The '496 patent discloses and claims a novel system including a switching valve arrangement to the inlet of a high pressure pump for delivery of liquid in an HPLC system.
The invention in the '496 patent overcomes limitations and disadvantages of the prior art by providing a system, suitable for use in liquid chromatography applications, in which problems associated with the non-uniformity of the draw stroke of an HPLC pump and fluid inertia of the solvent delivery system are minimized with a concomitant minimal impact on system design and cost. This is accomplished by connecting a plurality of reservoirs, each containing a liquid to be mixed to form a liquid composition, through a switching valve arrangement to the inlet of a high pressure pump for ultimate delivery to an HPLC column.
As described in the '496 patent, a flow and composition controller actuates the switching valves in a manner that allows the non-uniformity of the draw stroke of the pump to be equally shared by each of the liquid components over several cycles of switching valve actuation. A microprocessor drives both the pump and the fluid switching valves and includes means for generating a ratio between the time to connect all of the reservoirs selected for actuation and the cycle time for a pump draw stroke. By connecting the output of the pump drive to the valve drive, this ratio is held constant throughout the operation of the chromatographic system within a given flow range.
To avoid problems associated with the non-uniformity of the volumetric intake rate, according to the '496 patent, it had been found advantageous to establish the relationship, i.e. ratio, between the pump cycle time and the switching valve cycle time as a non-integer ratio that could be either greater than or less than one. The result was an averaging of the non-uniformity of the pump intake rate over many switching valve cycles, effecting compositional averaging that produced a more accurate mixture after several draw strokes.
The implementation of the non-integer ratio relationship between the pump cycle time and the switching valve cycle time created other concerns related to the affects on compositional accuracy resulting from the valves being actuated at widely varying pump intake rates. In particular, switching a valve open or closed during the rapid intake portion of the pump draw stroke, ideally, should cause instantaneous changes in flow through the valve. However, hydraulic inertia associated with the fluid in the relatively long length of tubing between the solvent reservoir and the valve resisted instantaneous flow changes. This resulted in inaccurate and unpredictable compositions which were highly dependent on specific operating conditions such as flow rate, tubing length, tubing diameter, tubing stiffness, solvent density, solvent compressibility, etc. These problems became magnified at high flow rates and/or short duration valve actuations, particularly in systems designed to operate over wide dynamic ranges of flow rate and solvent composition.
According to the '496 patent, and as illustrated in FIG. 1 herein (corresponding to FIG. 5 of the '496 patent with the reference numerals in accordance therewith), the hydraulic inertia related problems were addressed by implementing a series of hydraulic accumulators, one for each reservoir, directly adjacent to the switching valve arrangement 17A, 17B on the side closest to the reservoirs, i.e. the low pressure side of the system. The accumulators 19A, 19B allowed the fluid flow through the valves to accurately correspond to the rate of volume displacement during the pump draw stroke. Each accumulator 19A, 19B consisted of a soft-walled, flexible plastic tube 50 of generally circular cross-section. The accumulator tube 50 was adapted at an end closest to the valve inlet to snugly slide over a rigid plastic connector 52. A connecting tube 54 was implemented at the opposite end of the accumulator tube to hold the relatively long length of flow tubing 56 that connects the valves with the reservoirs. The end of the accumulator tube adjacent the connecting tube was caused to assume approximately the cross-section of a flattened ellipse 55 which allowed a significant internal volume change to occur in the accumulator tube, with little change in pressure thereby allowing the accumulator to overcome the effects of hydraulic inertia.
However, the implementation of such accumulator tubes is not without its own set of problems. The addition of accumulator tubes significantly increases the amount of space required to house the valve switching assembly. Sufficient space has to be provided to accommodate the tubes unhindered, e.g. without kinks or other restrictions.
The accumulator tubes each require interconnections at two ends, which presents the potential for unreliable sealing in the system. Inadequate seals at either end of the tubes results in leaks in the system for fluid to escape or for air to enter the system.
In addition to the potential for air to permeate the system at the exposed sealing points of the accumulator tubes, the large surface area of the tubes themselves are permeable, permitting air to enter the system. Air permeating the system and entering the solvent will cause pump flow errors. The problem created by the permeability of the tubes is exacerbated in systems where in-line solvent degassing is effected.
Furthermore, the accumulator tubes present areas for the collection of unswept volumes, that is, volumes of solvent that are not easily swept from the accumulator tubes. Unswept volumes will retain air bubbles, reducing the accuracy of the flow performance. Unswept volumes result in residual solvent components that require more time to clear the system and change between solvents. The accumulator tubes present the occurrence of unswept volumes which are difficult to eliminate, especially at the tubing joints.